It is desirable to improve gas turbine performance and improve overall specific fuel consumption (SFC) in a gas turbine. In a typical gas turbine, air is compressed through one or more compressors before combustion. The combustion products are expanded, and power is extracted therefrom, via one or more turbine stages. The combustion products are expelled at generally high temperature.
Generally during the compression process, the incoming air experiences significant heating during its reduction in specific volume, to the extent that material temperature limits may be reached. The overall pressure ratio (OPR) may be limited by the temperature limitations of the compressor itself. Thus, in order to improve the OPR and thereby the overall engine performance and SFC, known systems include providing one or more intercoolers that cool the compressed air between compressor stages. That is, by intercooling the compressed incoming air between stages, the air may be caused to achieve yet higher OPR while staying within the temperature limits of the compressor materials.
Known systems for intercooling include using air as the coolant between compressor stages. That is, air passing from a first stage compressor may pass through an air-air heat exchanger (i.e., an intercooler), resulting in cooled/compressed air that is passed to a second stage compressor where the compressed air is compressed to a yet higher pressure. Without the intercooler, compressed air exiting the second stage compressor may be excessive and may exceed acceptable temperature limits.
However, using air in an intercooler presents a number of challenges in gas turbine design. First, because of the relatively low thermal carrying capacity of air, adequate ducting should be provided such that enough mass flow is achieved to extract the energy from the compressed air after the first stage of compression, and reduce the compressed air to its inlet temperature to the second compression stage. Also, because the heat transfer fluid in the intercooler is air, separate air handling units are also provided that move the air in a cross or counter flow direction. Ducting and fans that move the air through the intercooler tend to be large, complex, and costly. An aircraft engine is in an environment that is sensitive to the amount of mass (i.e., weight in an aircraft application) and space is a premium.
In addition, heat removed between compression stages in an intercooler is typically ejected as waste heat and no work is extracted therefrom. That is, hot air from the intercooler is a low grade energy which presents little opportunity for work extraction, and it is simply ejected. Thus, in addition to providing compressor intercooling, there is also a need to reduce the spatial requirements and complexity of the turbine engine, while improving the overall SFC and energy output of the turbine engine.